1. Field of the Invention
The present invention relates to a variable speed generator-motor apparatus, and more particularly to a variable speed generator-motor apparatus provided with economical protection measures against grounding or short-circuiting fault occurring therein.
2. Description of the Related Art
A generator-motor apparatus of the alternating current (AC) excitation type, which is often used in a variable speed pumping-up electric power plant, comprises a generator-motor having an armature winding and a field winding provided on a stator and a rotor thereof, respectively, a main transformer connected to an electric power system, a synchronizing breaker provided between the main transformer and the armature winding of the generator-motor, a frequency converter, such as a cycloconverter, producing AC voltage of a low frequency for excitation of the field winding, a converter transformer for feeding the frequency converter, and an excitation breaker provided between the converter transformer and a junction of the main transformer and the synchronizing breaker.
Incidentally, in the following description, when only a generator-motor is referred to, distinguished from the whole of a generator-motor apparatus, it will be called an electric rotating machine or simply a rotating machine.
The rotor of the electric rotating machine is mechanically coupled to a water pump-turbine capable of operating selectively as a prime mover or a load. When driven by the pump-turbine, which operates as a water turbine, the rotating machine functions as a generator and supplies electric power to the power system through a main circuit of the generator-motor apparatus composed of the main transformer and, the synchronizing breaker. On the other hand, when the rotating machine is supplied by the power system with electric power, it operates as a motor and drives the pump-turbine so that it functions as a water pump.
In the generator-motor apparatus as mentioned above, by exciting the field winding with an AC voltage of the frequency depending on the difference between a synchronous speed determined by the frequency of the power system and a rotational speed of the rotor, the frequency of an output voltage of the electric rotating machine functioning as a generator can be maintained constant at the frequency of the power system free from fluctuation in a rotational speed of the pump-turbine functioning as a water turbine.
Similarly, in the case where the rotating machine operates as a motor, a rotational speed of the pump-turbine operating as a pump can be arbitrarily controlled irrespective of the fixed frequency of voltage applied to the rotating machine from the power system. As a result, the pump-turbine can be most efficiently operated at the adaptive speed both in the motor operation and in the generator operation.
A generator-motor apparatus applied to a variable speed pumping-up electric power plant, is disclosed in copending U.S. patent application Ser. No. 044,404 (filed on Apr. 30, 1987), now U.S. Pat. No. 4,816,696 entitled "Variable-speed Pumped-storage Power Generating System".
According thereto, a main circuit of a generator-motor apparatus includes a main transformer and a synchronizing breaker connected in series thereto. Further, the main circuit is formed between an armature winding of an electric rotating machine and an electric power system and communicates electric power therebetween.
An excitation circuit for a field winding of the rotating machine includes a cycloconverter and a converter transformer. The cycloconverter is fed by the converter transformer and supplies the field winding with AC voltage of the variable frequency. The converter transformer is connected to a junction of the main transformer and the synchronizing breaker through an excitation breaker. As will be apparent from the description above, therefore, the synchronizing breaker and the excitation breaker are provided in parallel with each other with respect to the power system.
With the disposition of the excitation breaker as mentioned above, the following problem is developed. Namely, an excitation breaker is required to have a current breaking capacity greater than that of a synchronizing breaker. For example, the synchronizing breaker is required to cut off a fault current incoming to a rotating machine from a power system, if grounding or short-circuiting fault occurs on the side of the rotating machine with respect to the breaker, and cut off a fault current flowing toward the power system from the rotating machine, if such a fault occurs on the side of a main transformer or the power system with respect to the breaker.
On the other hand, the excitation breaker must cut off a composite fault current from both sides of the power system and the rotating machine, in the case of occurrence of the grounding or short-circuiting fault in the excitation circuit. Therefore, the current to be cut off by the excitation breaker becomes substantially double the current to be cut off by the synchronizing breaker. Accordingly, as the excitation breaker, there is required an expensive breaker with a large current breaking capacity, with the result that the protection measures for a generator-motor apparatus becomes expensive as a whole.